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Documentation / ia64 / paravirt_ops.txt




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Based on kernel version 3.16. Page generated on 2014-08-06 21:39 EST.

1	Paravirt_ops on IA64
2	====================
3	                          21 May 2008, Isaku Yamahata <yamahata@valinux.co.jp>
4	
5	
6	Introduction
7	------------
8	The aim of this documentation is to help with maintainability and/or to
9	encourage people to use paravirt_ops/IA64.
10	
11	paravirt_ops (pv_ops in short) is a way for virtualization support of
12	Linux kernel on x86. Several ways for virtualization support were
13	proposed, paravirt_ops is the winner.
14	On the other hand, now there are also several IA64 virtualization
15	technologies like kvm/IA64, xen/IA64 and many other academic IA64
16	hypervisors so that it is good to add generic virtualization
17	infrastructure on Linux/IA64.
18	
19	
20	What is paravirt_ops?
21	---------------------
22	It has been developed on x86 as virtualization support via API, not ABI.
23	It allows each hypervisor to override operations which are important for
24	hypervisors at API level. And it allows a single kernel binary to run on
25	all supported execution environments including native machine.
26	Essentially paravirt_ops is a set of function pointers which represent
27	operations corresponding to low level sensitive instructions and high
28	level functionalities in various area. But one significant difference
29	from usual function pointer table is that it allows optimization with
30	binary patch. It is because some of these operations are very
31	performance sensitive and indirect call overhead is not negligible.
32	With binary patch, indirect C function call can be transformed into
33	direct C function call or in-place execution to eliminate the overhead.
34	
35	Thus, operations of paravirt_ops are classified into three categories.
36	- simple indirect call
37	  These operations correspond to high level functionality so that the
38	  overhead of indirect call isn't very important.
39	
40	- indirect call which allows optimization with binary patch
41	  Usually these operations correspond to low level instructions. They
42	  are called frequently and performance critical. So the overhead is
43	  very important.
44	
45	- a set of macros for hand written assembly code
46	  Hand written assembly codes (.S files) also need paravirtualization
47	  because they include sensitive instructions or some of code paths in
48	  them are very performance critical.
49	
50	
51	The relation to the IA64 machine vector
52	---------------------------------------
53	Linux/IA64 has the IA64 machine vector functionality which allows the
54	kernel to switch implementations (e.g. initialization, ipi, dma api...)
55	depending on executing platform.
56	We can replace some implementations very easily defining a new machine
57	vector. Thus another approach for virtualization support would be
58	enhancing the machine vector functionality.
59	But paravirt_ops approach was taken because
60	- virtualization support needs wider support than machine vector does.
61	  e.g. low level instruction paravirtualization. It must be
62	       initialized very early before platform detection.
63	
64	- virtualization support needs more functionality like binary patch.
65	  Probably the calling overhead might not be very large compared to the
66	  emulation overhead of virtualization. However in the native case, the
67	  overhead should be eliminated completely.
68	  A single kernel binary should run on each environment including native,
69	  and the overhead of paravirt_ops on native environment should be as
70	  small as possible.
71	
72	- for full virtualization technology, e.g. KVM/IA64 or
73	  Xen/IA64 HVM domain, the result would be
74	  (the emulated platform machine vector. probably dig) + (pv_ops).
75	  This means that the virtualization support layer should be under
76	  the machine vector layer.
77	
78	Possibly it might be better to move some function pointers from
79	paravirt_ops to machine vector. In fact, Xen domU case utilizes both
80	pv_ops and machine vector.
81	
82	
83	IA64 paravirt_ops
84	-----------------
85	In this section, the concrete paravirt_ops will be discussed.
86	Because of the architecture difference between ia64 and x86, the
87	resulting set of functions is very different from x86 pv_ops.
88	
89	- C function pointer tables
90	They are not very performance critical so that simple C indirect
91	function call is acceptable. The following structures are defined at
92	this moment. For details see linux/include/asm-ia64/paravirt.h
93	  - struct pv_info
94	    This structure describes the execution environment.
95	  - struct pv_init_ops
96	    This structure describes the various initialization hooks.
97	  - struct pv_iosapic_ops
98	    This structure describes hooks to iosapic operations.
99	  - struct pv_irq_ops
100	    This structure describes hooks to irq related operations
101	  - struct pv_time_op
102	    This structure describes hooks to steal time accounting.
103	
104	- a set of indirect calls which need optimization
105	Currently this class of functions correspond to a subset of IA64
106	intrinsics. At this moment the optimization with binary patch isn't
107	implemented yet.
108	struct pv_cpu_op is defined. For details see
109	linux/include/asm-ia64/paravirt_privop.h
110	Mostly they correspond to ia64 intrinsics 1-to-1.
111	Caveat: Now they are defined as C indirect function pointers, but in
112	order to support binary patch optimization, they will be changed
113	using GCC extended inline assembly code.
114	
115	- a set of macros for hand written assembly code (.S files)
116	For maintenance purpose, the taken approach for .S files is single
117	source code and compile multiple times with different macros definitions.
118	Each pv_ops instance must define those macros to compile.
119	The important thing here is that sensitive, but non-privileged
120	instructions must be paravirtualized and that some privileged
121	instructions also need paravirtualization for reasonable performance.
122	Developers who modify .S files must be aware of that. At this moment
123	an easy checker is implemented to detect paravirtualization breakage.
124	But it doesn't cover all the cases.
125	
126	Sometimes this set of macros is called pv_cpu_asm_op. But there is no
127	corresponding structure in the source code.
128	Those macros mostly 1:1 correspond to a subset of privileged
129	instructions. See linux/include/asm-ia64/native/inst.h.
130	And some functions written in assembly also need to be overrided so
131	that each pv_ops instance have to define some macros. Again see
132	linux/include/asm-ia64/native/inst.h.
133	
134	
135	Those structures must be initialized very early before start_kernel.
136	Probably initialized in head.S using multi entry point or some other trick.
137	For native case implementation see linux/arch/ia64/kernel/paravirt.c.
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